1. Field of the Invention
This invention relates to improvements in phase modulated communications systems, and, more particularly to improvements in error alarms used in conjunction with phase locked signal sources for use in phase modulated communications systems.
2. Description of the Prior Art
In microwave telecommunications, the transmission of digital voice signals is becoming of great interest. Typically, digital microwave telecommunications are achieved with a phase modulated signal which includes a number of voice channels multiplexed into a single channel for transmission over the distances required. Upon reception of the signal, the various individual signals are demultiplexed and distributed to their appropriate destinations. In such data communications, however, precision of the appropriate phase modulation must be flawlessly maintained in order that the signals can be properly detected and decoded at various points along the distribution route.
As background for understanding the present invention, the mechanics of a phase modulated system are described. In a typical phase modulated signal, the phase is changed with reference to a carrier frequency, the change in phase being detected as the data. In order to determine the phase change, most phase modulation signal receivers include a carrier recovery loop which generates a reference carrier signal from the received signal and against which changes in phase in the received signal are compared. In a four phase system, for example, if the phase changes of the carrier are 0, 90, 180, and 270 degrees, each of the four phase angles indicates a particular data state or data state change, as is well known in the art. Because of variations throughout the transmission, however, the actual phase states determined by the receiver might not coincide precisely with one of the four phase coordinates. For example, a phase state intended to correspond to a 90 degrees phase shift may in fact be detected as only an 85 degree phase change. This is not important to most receivers, since typical phase modulation signal receivers automatically align a detected phase change with the closest phase pole. Thus, for example, in a four phase system, so long as a phase change is within plus or minus 45 degrees of the intended phase, the receiver will automatically lock (detect, decode, or attribute) the data onto the appropriate closest phase pole. It can be seen, however, that if the error exceeds 45 degrees, the data will be interpreted as being a different phase from that actually intended, and a data detection error will occur.
In many receivers, the change in the frequency of the reference carrier developed by the carrier recovery loop may be permitted to gradually drift or change without affecting the accuracy of the data detection process of the receiver. This is because a phase locked loop is typically provided in the receiver to follow the carrier recovery signal. Therefore, the main carrier can be allowed to drift, within limits, slowly within a certain range of carrier frequencies so long as the range is within the lock-in range of the phase locked loop of the receiver.
If, however, the transmitted carrier experiences an instantaneous or rapid phase jump, the frequency of the reference signal developed by the carrier recovery loop of the receiver will not fully or immediately follow the instantaneous jump. Therefore, data on the received signal will be compared against the relatively stable signal generated by the carrier recovery loop, it will be of different phase from that intended to be transmitted and will be erroneously detected.
In microwave technology, tuning is often affected by changes in mechanical parts of the system, such as thermally or mechanically induced change in length of a transmission line, the tuning of a microwave horn, or the like. The making or breaking of an electrical contact can occur by slow mechanical changes, but which can result in rapid electrical changes. Changes in the position of an effective contact point can easily shift the frequency of microwave oscillators. The transmission of the signal may therefore be flawed by a momentary "hit", or "glitch" caused by an unintended mechanical movement of one or more of the system parts. The phase of the transmitted or received signal may be therefore be undesirably affected. If the onset of the "hit" is more rapid than about one millisecond, the microwave receiver may be unable to distinguish the momentary unwanted "hit" caused phase change from a desired data phase change, thereby producing errors in the detected data. This is because the receiver carrier recovery phase lock loop has a limited ability to follow changes in the incoming carrier frequency. Data errors caused by this "hit" induced noise phase may disturb the receiver multiplex "framing system", in which case various received signals may be erroneously and chaotically distributed to the ultimate distribution points. It is therefore extremely important that the microwave frequency sources be maintained as steady as possible to achieve reliable communications. Because mechanical elements and other uncontrollable circuit parts are involved, total correction of the problem is not entirely possible, at present. This uncertainty requires that the errors at least be detected to warn of the existance of possible errors in the detected signal.
However, since microwave communications are conducted over a number of relay stations between a point of origin and a point of destination, when a phase "hit" or "glitch" occurs, because of the very fact that it is momentary, less then one millisecond, in many instances, although the diagnosis of the fact of the occurrence of a major error or "hit" is obvious because of the condition of the data, the localization of the error to a particular part or module is practically impossible without an effective alarm system.
In various microwave transmitters and receivers, a frequency source is generally employed by which the appropriate signal transmission or detection frequencies are established. Typical sources usually include a crystal controlled oscillator and a voltage controlled oscillator (VCO), which each generate frequencies for comparison by such as a mixer or the like to produce a phase error signal. The error signal in such systems, is fed back to the VCO to control its frequency, and thereby the phase difference between the VCO output and the crystal controlled oscillator output (phase being the integral of frequency) are maintained in phase lock. The output of the VCO typically is used to provide the microwave frequency desired for controlling the transmission or reception of the signal (in some cases after being multiplied by an appropriate factor).
In the past, the primary way for detecting the occurrence of an error in the source has been to monitor the voltage excursions applied to the input of the VCO from the comparing mixer. When the voltage approaches some predetermined limit beyond which the VCO cannot be properly phase-locked, an alarm is initiated. In such systems, however, the alarm is indicated only upon the occurrence of gross or long term errors; that is, when the controlled voltage has continued to its practical phase locking operating limit, an alarm is indicated, but an alarm is not indicated upon small, momentary interruptions on the order of less than one millisecond. Although the long-term interruptions are important, as described above, the momentary or instantaneous events can result in as much chaos in the demodulation of the signal as an out of lock phase locked loop.
It is desired, of course, to not only be informed of the occurrence of an error, but ultimately it is desirable to isolate the cause of the error so that the particular error causing conditions can be corrected. However, because the errors may be of such short duration, or are often times intermittent because of the mechanical conditions above described, or for some other reason, the isolation of the source of the errors is extremely difficult to identify. And, as above described, the alarm systems of the prior art which produce alarms only upon prolonged and large error producing events make the isolation and identification of the error producing part or system virtually impossible.
In general, the prior art source alarm circuits are basically phase lock alarms. The alarm operates when the VCO control voltage reaches or approaches the end of the usable phase lock range. However, it is possible for a "hit" induced phase error to occur that will put errors into the data detected at the receiver without causing the prior art phase lock alarm type circuits to alarm. Experience has shown that data errors can be induced in the receiver by such "hit" induced source phase errors which create only small and short duration change in the VCO control voltage. On the other hand, large changes in the VCO control voltage can occur without causing errors in the data detected by the receiver.
Commonly, prior art sources have a sweep circuit that causes the VCO control voltage to sweep over the full control range in the attempt to acquire phase lock. Some prior art alarm circuits detect (rectify) this large low frequency sweep signal and operate the alarm in response to its presence.